The present invention relates to a fuel injection device of a direct injection engine.
For example, EP2302197A1 (JP2010-019194A) discloses a fuel injection valve having first and second solenoid coils to serve as a solenoid-operated fuel injection valve. Specifically, the fuel injection valve disclosed in EP2302197A1 (JP2010-019194A) has a first solenoid coil for allowing the valve body to stroke within a relatively large range, and a second solenoid coil for allowing the valve body to stroke within a relatively small range. When the engine load is low, the stroke range of the valve body is reduced by supplying power only to the second solenoid coil so as to perform a stratified lean combustion. On the other hand, when the engine load is high, the stroke range of the valve body is increased by supplying the power only to the first solenoid coil so as to perform a homogeneous combustion at λ=1.
Meanwhile, a combustion mode for compressing lean mixture gas for ignition has been known as an art of achieving both improvements in exhaust emission performance and thermal efficiency. Increasing a geometric compression ratio of an engine where such compression-ignition combustion is performed leads to an increase in pressure and temperature of the end of compression stroke, and thus it is advantageous in stabilizing the compression-ignition combustion.
However, compression-ignition combustion generally becomes pre-ignition combustion with significant pressure increase as the engine load increases. Therefore, it causes an increase in combustion noise and abnormal combustion (e.g., knocking), as well as increase in Raw NOx due to a high combustion temperature. Thus, even with an engine in which compression-ignition combustion is performed, within a high-engine-load side operating range, spark-ignition combustion has generally been performed by operating an ignition plug instead of the compression-ignition combustion. However, with an engine which has its geometric compression ratio set high to stabilize the compression-ignition combustion, abnormal combustion (e.g., pre-ignition or knocking) is caused within the high-engine-load operating range.
In this regard, the applicant of the present invention obtained knowledge that within the high-engine-load operating range, it is effective, in avoiding such abnormal combustion, to inject fuel into a cylinder at a comparatively high fuel pressure at a timing near a compression top dead center because such an injection shortens an injection period, a mixture gas forming period, and a combustion period. The avoidance of abnormal combustion contributes in improving fuel consumption within the high-engine-load range where the spark-ignition combustion is performed. In achieving such fuel injection mode, it is required to increase an injection rate (i.e., the injection amount per time unit), and as a method for the increase, the stroke amount of the valve body of the fuel injection valve may be increased.
However, the large stroke amount of the valve body will cause a new problem of degradation in accuracy of controlling the injection amount within an operating range where the fuel injection amount is set less, such as a low-engine-load operating range.
The present invention is made in view of the above situation, and aims to achieve fuel consumption improvement over a wide operating range of an engine by using a fuel injection valve improved in fuel injection accuracy over a wide range between a low injection amount to a high injection amount.